This invention relates to novel blocked polyisocyanates that are soluble or dispersible in water and which contain phenolate anions in a chemically bound form as groups which provide their solubility or dispersibility. It also relates to a process for the preparation of these blocked polyisocyanates in the form of a solution or dispersion in water, optionally in the presence of coreactants, and to their use as hardeners in two-component lacquers, binders, or adhesive formulations.
Water-dispersible blocked polyisocyanates are known as disclosed by U.S. Pat. Nos. 4,098,933 and 4,284,544. Their primary utility has, however, been as cross-linkers for aqueous polyurethane dispersions, polyester polyols or polyhydroxy polyacrylates. The compounds mentioned in these publications may be used in aqueous solution or dispersion, inter alia in combination with water-dispersible or water-soluble polyhydroxyl compounds as binders for polyurethane lacquers that are applied from the aqueous phase. If, however, the polyhydroxyl compounds used are not normally soluble in water, the two components must first be modified in separate reaction mixtures to render them hydrophilic and then emulsified in water with the aid of organic cosolvents, and the two reaction mixtures are then mixed together. In the processes described in the aforesaid prior publications, moreover, a solvent-free preparation of water-dispersible or water-soluble blocked polyisocyanate is possible only with the addition of large quantities of ionic starting components, or large proportions of polyethers containing ethylene oxide units as hydrophilic reaction components. This has disadvantageous effects on the resistance to water and stability to light of the surface structures finally obtained.
Thus, in the process for the preparation of the known water soluble or dispersible blocked polyisocyanates, a significant portion of the isocyanate groups present in the polyisocyanates used as starting material is used up by the hydrophilic modification. The cross-linking capacity of the polyisocyanates is thereby considerably reduced. Accordingly, it is one object of the present invention to provide water soluble or dispersible polyisocyanates in which the hydrophilic group which renders the polyisocyanates soluble or dispersible takes part in the cross-linking reaction so that it becomes incorporated in the polyisocyanates and thereby preserves their cross-linking potential.
This object can be achieved by providing the blocked polyisocyanates as described in, for example, U.S. Pat. Nos. 4,522,851 and 4,608,304. The incorporated carboxylic acid groups therein are rendered hydrophilic by neutralizing them with volatile amine compounds. When the coating cures, the amine compound is removed from the coating and the freed carboxylic acid, besides being less hydrophilic, can further react with epoxide groups included in the formulation. Thus, additional crosslinking sites are provided. However, this technology is limited to those cases wherein the formulations contain aqueous resins with epoxide groups. In many cases, these epoxide resins have also been made hydrophilic by the incorporation of polyethylene oxide so that the overall hydrophilicity is increased.
This object can also be achieved by providing the blocked polyisocyanates according to U.S. Pat. Nos. 4,144,268 and 4,284,544. The hydrophilic carbamoyl sulfonate groups present in the blocked polyisocyanates according to these patents also serve a double function. They ensure that the polyisocyanates are soluble or dispersible in water and also take part in the crosslinking reaction at the same time losing their hydrophilic character. The low pH of aqueous solutions of these blocked isocyanates (ca. 3-6), however, limits their usefulness to systems with neutral or lower pH values.
Another method which yields novel blocked polyisocyanates, which are soluble or dispersible in water, is described in, for example, U.S. Pat. Nos. 4,619,966 and 4,895,921. Cyanurea anions are chemically bound with polyisocyanates to function as groups that ensure their solubility or dispersibility. These blocked polyisocyanates can be used in the form of a solution or dispersion in water as hardeners in two-component lacquers. However, due to the low reactivity of dicyanamide anion with isocyanates, it is not possible to prepare these blocked isocyanates directly in aqueous solution. Thus, the aqueous dispersions of the blocked polyisocyanates must be formed in a second step.
Methods for the direct preparation of blocked polyisocyanates in aqueous media are known and described in, for example, U.S. Pat. No. 5,296,160. These solutions can even be prepared in the presence of polyhydroxyl compounds as is described in, for example, U.S. Pat. No. 5,300,556, when the appropriate blocking agents are selected. However, the polyisocyanates must be modified to be dispersible in water, and therefore have lower crosslinking potential than ones wherein the blocking agent provides hydrophilicity.
Surprisingly, it has been found that novel blocked polyisocyanates that are soluble or dispersible in water and contain phenolate anions as groups that provide their solubility or dispersibility can be prepared directly in aqueous media. Processes for the preparation of these blocked polyisocyanates in the form of a solution or dispersion in water, optionally in the presence of coreactants, as well as their use as hardeners in two-component lacquers, binders, or adhesive formulations, has also been found. In addition to being compatible with the highly alkaline aqueous resins, they can serve as crosslinking sites for systems with neutral or higher pH values.